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1By Diptarshi BhowmickBy Diptarshi Bhowmick1INTRODUCTION Stepper motors are highly accurate pulse-driven motors that change their angular position in steps, in response to input pulses from digitally controlled systems.

The stepper motor makes a step for each applied pulse.

The size of the step (step angle) depends on the type and design of the stepper motor.

The input pulses to the stepper motor must be in a proper sequence with acceptable frequency and must provide the phase windings with sufficient current.

Typical applications of stepper motors requiring incremental motion are printers, disk drives, robotics, X-Y plotters.2By Diptarshi Bhowmick2By Diptarshi BhowmickOperating principalPrinciple of Alignment

Pieces of highly permeable materials such as iron, situated in an ambient medium of low permeability such as air in which magnetic field is established, experience mechanical forces that tend to align them in such a way to minimize the reluctance of the system.

3By Diptarshi Bhowmick3By Diptarshi BhowmickBy Diptarshi Bhowmick4The stepper motor uses the theory of operation for magnets to make the motor shaft turn a precise distance when a pulse of electricity is provided. You learned previously that like poles of a magnet repel and unlike poles attract. Figure 1 shows a typical cross-sectional view of the rotor and stator of a stepper motor. From this diagram you can see that the stator (stationary winding) has eight poles, and the rotor has six poles (three complete magnets).

Operating principalBy Diptarshi Bhowmick5The rotor will require 24 pulses of electricity to move the 24 steps to make one complete revolution. Another way to say this is that the rotor will move precisely 15 for each pulse of electricity that the motor receives. The number of degrees the rotor will turn when a pulse of electricity is delivered to the motor can be calculated by dividing the number of degrees in one revolution of the shaft (360) by the number of poles (north and south) in the rotor. In this stepper motor 360 is divided by 24 to get 15.

Operating principal(Contd)By Diptarshi Bhowmick6Step angle calculationStator pole pitch;Rotor pole pitch;Step angle;

Operating principal(Contd)By Diptarshi Bhowmick7

This figure shows the movement of the stepper motor rotor as current is pulsed to the stator. (a) Current is applied to the A and A windings, so the A winding is north, (b) Current is applied to B and B windings, so the B winding is north, (c) Current is applied to the C and C windings, so the C winding is north, (d) Current is applied to the D and D windings so the D winding is north. (e) Current is applied to the A and A windings, so the A winding is north.Operating principal(Contd)By Diptarshi Bhowmick8The stepper motor can be operated in three different stepping modes, namely, full-step, half-step, and microstep.

Full-StepThe stepper motor uses a four-step switching sequence, which is called a full-step switching sequence which is already described above. Here only one pair of winding is energized at a time.

Stepper Motor Switching Sequence

By Diptarshi Bhowmick9Another switching sequence for the stepper motor is called an eight-step or half-step sequence. The switching diagram for the half-step sequence is shown in Fig. The main feature of this switching sequence is that you can double the resolution of the stepper motor by causing the rotor to move half the distance it does when the full-step switching sequence is used. This means that a 200-step motor, which has a resolution of 1.8, will have a resolution of 400 steps and 0.9. The half-step switching sequence requires a special stepper motor controller, but it can be used with a standard hybrid motor. The way the controller gets the motor to reach the half-step is to energize both phases at the same time with equal current.Half-StepStepper Motor Switching SequenceBy Diptarshi Bhowmick10

This shows the switching sequence for the eight-step input (half-step mode). The sequence continues for eight steps and then repeats. The main difference between this sequence and the full-step sequence is that the energizing sequence for half step is A AB B BC C CD D DA & switching sequences will be SW1, SW1 & SW2, SW2, SW2 & SW3, SW3, SW3 & SW4, SW4 and so on.Stepper Motor Switching SequenceHalf-StepBy Diptarshi Bhowmick11Micro Step ModeThe full-step and half-step motors tend to be slightly jerky in their operation as the motor moves from step to step. The amount of resolution is also limited by the number of physical poles that the rotor can have. The amount of resolution (number of steps) can be in-creased by manipulating the current that the controller sends to the motor during each step. The current can be adjusted so that it looks similar to a sine wave

Stepper Motor Switching SequenceBy Diptarshi Bhowmick12This figure shows the waveform for the current to each phase.

Current to each individual phase increases and decreases like a sine wave and that is always out of time with the other phase will allow the rotor to reach hundreds of intermediate steps.In fact it is possible for the controller to reach as many as 500 micro steps for a full-step sequence, which will provide 100,000 steps for each revolution.

Micro Step ModeCurrent sent to each of the four sets of windings is timed so that there is always a phase difference with each other.Stepper Motor Switching SequenceTYPES OF STEPPER MOTORSVariable reluctanceHybridPermanent MagnetSingle-stackSSVRMulti-stackMSVR13By Diptarshi BhowmickVARIABLE RELUCTANCE (Contd)Single-Stack Variable Reluctance (SSVR)By Diptarshi Bhowmick14

Modes of operation:This figure shows the mode of operation for a 45 step in clockwise direction. The winding are energized in the sequence A, A+B, B, B+C, and so forth and the sequence is repeated. The direction of rotation can be reversed by reversing thesequence of switching the windingsBy Diptarshi Bhowmick15VARIABLE RELUCTANCE (Contd)Single-Stack Variable Reluctance (SSVR)3-phase, 4-pole VR Motor

Phases 1, 2 and 3 are energized in sequence making the rotor rotating in steps of 30o.By Diptarshi Bhowmick16

VARIABLE RELUCTANCE (Contd)Single-Stack Variable Reluctance (SSVR)